The present invention relates to a technique for producing colored electroplated nickel coatings and to articles producible by this technique.
The electrodeposition of nickel on metal substrates such as steel, copper and brass, is widely used in industry in order to meet both decorative and protective requirements for a wide range of goods. The properties provided by an electrodeposited nickel surface, for engineering applications, are generally adhesion, and corrosion- and wear-resistance, hardness and ductility, while for consumer applications the same qualities are relevant, and additionally the appearance of the surface becomes of great importance as part of the decorative value of the products.
The appearance of an electrodeposited nickel coating is usually described in terms of properties such as brightness, reflectivity, tarnish resistance, smoothness, texture and so forth. For esthetic reasons, the color of the coating is also of importance, especially for consumer applications, but the possibilities for imparting intrinsic color to electrodeposited nickel are very limited.
While aluminum may be provided with an oxide film coating which imparts excellent corrosion- and wear-resistance, by an electrolytic process in which aluminum constitutes the anodexe2x80x94xe2x80x9canodizingxe2x80x9dxe2x80x94and while such a coating may be successfully colored, such a technique is not applicable to nickel.
In painting technology, it is known to provide surfaces with pigmented polymeric coatings, in order to obtain articles with a colored finish, but of course the surface is not metallic, and thus cannot for example be selected to be a mirror, matt, full-bright or semi-bright finish. Moreover, the manufacturing process then requires an additional coating-pigmenting step, which it would be desirable to avoid, if this were possible.
It is also known to provide colored metallic finishes on (usually bright) electrodeposited nickel with a restricted range of colors. Thus, various hues and shades of gold can be deposited in this manner from gold cyanide electrolyte, and silver can be plated from cyanide electrolyte from a dissolving silver anode. Similarly, a dark gray-blue finish can be imparted to nickel by electrodeposited ruthenium. Such metallic finishes suffer from the following drawbacks:
(a) the color range is limited to golds, silvers and gray-blues;
(b) the high price of the coloring component makes such processes expensive, and in case stripping is required this would also be expensive;
(c) plating from cyanide electrolytes is neither user-friendly nor environment-friendly;
(d) each color requires its own special electrolyte, so that the plating bath must be changed in order to change the color.
In an attempt to meet in particular the limitation of the narrow range of obtainable colors, a number of formulations have been developed for coloring metal surfaces electrolytically or by dipping. By way of example, a solution of lead acetate, sodium thiosulfate and acetic acid can produce a blue color on electrodeposited nickel; a solution of potassium chlorate, and copper and nickel sulfates can produce brown colors on brass and copper; and a solution of copper sulfate containing acetic acid and glycerol, in addition to ammonium, sodium and zinc chlorides, produces the so-called tiffany green on brass or nickel, by repeated immersion and drying of the articles in question. Production of such single colors is unlikely to be economical, and it should also be noted that similarly to the previously-mentioned overplating techniques using gold, silver or ruthenium, these colors each require particular process conditions and often exotic electrolytes or dipping solutions, so that the plating conditions and the bath must be changed in order to change the color, which features of course add to the difficulties of carrying out operations which are commercially viable. An additional problem in such cases is that the obtainable colors and hues are sensitive to slight changes in plating parameters, so that the results may depend more on the operator""s skill, than on a particular formulation and plating conditions.
Another approach to solving the problem of the lack of variety of colors available by simply overplating nickel, has been the electrophoretic technique, which involves the deposition of pigment particles in the micronic size range from a pigment suspension in an electroplating bath. Although this technique does provide a variety of colors in the articles thus produced, at the same time the finishes lack the brilliance of nickel-plated articles and are tarnish-like, semi-bright colors. As we have seen in various known techniques, here too, each color requires its special coloring bath, and changing the color means changing the bath. Moreover, stripping of the color is not practical, so that if the finished article is defective in color or appearance, the defect cannot be repaired.
Although not answering consumer demand for a variety of colors, electrodeposition on a metal cathode of a black coating known as xe2x80x9cblack oxidexe2x80x9d or xe2x80x9cblack nickelxe2x80x9d, is also commercially available, and affords a range from light gray to black anthracite. Black nickel is usually plated onto a brass or nickel base, or onto steel provided with an intermediate layer of zinc, copper or nickel. A variety of electroplating conditions and electrolyte formulations for such purposes have been described in the art, but the formulations virtually all contain zinc, nickel and sulfur, in thiosulfate. These formulations, generally termed xe2x80x9coxidizing liquidxe2x80x9d are available in the market, in concentrated liquid form. According to U.S. Pat. Nos. 4,861,441 and 5,011,744, black nickel coatings of excellent quality are said to be obtainable in presence of a strongly oxidizing anion, and cations of Zn and a xe2x80x9ccoloring metalxe2x80x9d i.e. Fe, Co, Ni, Cr, Sn or Cu, at a pH of 1-4, a current density of 5-100 A/dm2 and a current quantity of 20-200 coulombs/dm2. Somewhat similar are processes for obtaining a black electrodeposited coating, described in U.S. Pat. Nos. 4,968,391 and 5,023,146, in which the bath contains additionally a sulfur compound such as a thiocyanate or a thiosulfate, and the preferred current density is 1-50 A/dm2. Also described in the literature is a process for obtaining black nickel electroplated coatings from a bath containing Ni, Zn and ammonium cations and thiocyanate anions, at a pH of from 3.5 to 6.0, and a cathode current density of 0.15-0.2 A/dm2 (Dennis, J. K. and Such T. E., Nickel and Chromium Plating, 2nd Edition, Butterworth, 1986, see also U.S. Pat. No. 2,844,530).
A phenomenon related to the problem of providing electrodeposited colored metallic surfaces is that of light interference in submicronic/micronic electroplated films, in which the color depends on film thickness. For example, cuprous oxide changes its color from an initial violet through blue, green, yellow, orange and red, due to the interference phenomenon, as the film thickness increases (see e.g. Solomon, H., Isserlis, G. and Averil, A. F., xe2x80x9cProtective and Decorative Coatings for Metalsxe2x80x9d, Finishing Publications Ltd., USA, 1978). However, this phenomenon is not commercially viable because of the unreliability of the desired color, since the slightest changes in electroplating parameters or physical variation in the metal surface, leads to an even more dramatic change, in color or hue, of the electroplated film.
The entire contents of the above-mentioned patents and literature references are incorporated by reference herein. Briefly summarized, the need for a viable process for obtaining the unique and vivid beauty of mirror-like full-bright nickel coated metal surfaces in a variety of colors has not been satisfied by techniques known in the art.
A primary object of the invention is to provide a colored electroplated coating on bright or matt nickel as underplate, and a process for the preparation thereof.
Another object of the invention is to provide a colored electroplated coating, and a process for the preparation thereof as just recited, wherein the color of the coating can be varied and can be predetermined by selecting process parameters.
Still another object of the invention is to provide a colored electroplated coating as aforesaid, and a process for the preparation thereof, wherein the coating has a lustrous brilliant appearance similar to a high level conventional bright or matt electroplated nickel coating.
Yet another object of the invention is to provide a colored electroplated coating as aforesaid, and a process for the preparation thereof, wherein the ingredients of the electrolytes used are neither more expensive nor more hazardous than those used conventionally for nickel electroplating.
Yet a further object of the invention is to provide a colored electroplated coating as aforesaid, and a process for the preparation thereof, wherein the coloring process is stable, in that acceptable variation of colors can be assured by corresponding variation within a reliable range of process parameters.
Another very important object of the invention is to provide a colored electroplated coating as aforesaid, and a process for the preparation thereof, wherein various colors and hues of the colored coating can be produced using the same bath and the same electrolyte solution, by selecting the process parameters exclusively.
Other objects of the invention will be apparent from the description which follows.
In one aspect, the present invention provides an article including a colored electroplated metallic coating comprising both nickel and zinc, on bright or matt nickel as underplate, wherein variation of the color of the electroplated coating does not depend on variation of the identity of the cations in the electrolyte from which said coating is electrolytically deposited. This aspect of the invention as claimed herein is subject to a proviso that more than 65% of the colors in said coating are other than black and that the colors in said coating are not selected exclusively from the group consisting of black and white.
More particularly, in the article of the invention, the color of the electroplated colored coating has been preselected exclusively by variation of parameters in the electroplating step selected from current density, time of the electroplating step and current quantity, subject to the condition that a current density applied to the article as cathode was within the range of 0.01 to 0.5, preferably 0.02 to 0.2 A/dm2.
In another aspect, the invention provides a process for manufacturing an article with a colored metallic coating comprising both nickel and zinc, which process includes the step of electroplating said coating on bright or matt nickel as underplate, in an electrolyte bath at a pH in the range of 4.5 to 5.5 and at a temperature within the range of 15 to 35xc2x0 C., containing as cations Ni2+, Zn2+ and (NH4)+, and (SCN)xe2x88x92(thiocyanate) anions, wherein the color of the electroplated coating is preselected exclusively by variation of parameters selected from current density, time of the electroplating step and current quantity, subject to the condition that a current density is applied to said underplate as cathode within the range of 0.01 to 0.5, preferably 0.02 to 0.2 A/dm2.
For the purpose of the present specification and claims, the xe2x80x9ccolorxe2x80x9d of the electroplated coating has its ordinary dictionary meaning, excluding 100% black and/or white; thus, articles having wholly black and/or white electrodeposited coatings on a nickel underplate, and processes for manufacturing them, are excluded from the scope of the present invention. However, articles including mixtures of colors in the coating, and processes for manufacturing them, are included in the invention. In accordance with a preferred embodiment of the invention, more than 65% of the color or colors in the colored coating (e.g. according to the color analysis of the Pantone Guide) is/are other than black. Thus, in the examples (infra), which are of course only illustrative and do not limit the invention, in round figures, 66, 73, 80, 86, 89, 90, 93 or 100% of the color(s) in the colored coating are other than black when analyzed according to the Pantone Guide.
The terms xe2x80x9celectroplatedxe2x80x9d, xe2x80x9celectroplatingxe2x80x9d and similar terms have their normal meaning in the art and thus exclude, for example, other electrical processes such as electrophoresis.
From what has been stated above, the term xe2x80x9cvariation of parametersxe2x80x9d as used herein in relation to the process of the invention, or in relation to the process by which the article of the invention may be obtained, refers in particular to variation of current density, time of the electroplating step and current quantity (which of course are interrelated); such variation permits the obtainment of coatings of preselected colors. On the other hand, it is a particular and distinctive feature of the present invention that different colored coatings are obtained while maintaining the identity of the chemical ingredients in the electroplating bath. Thus, according to the invention, selection of the color of the coating is not determined by adding or subtracting ingredients in the electroplating step. The electroplating step is also preferably carried out within prescribed ranges of pH and temperature.
It will accordingly be apparent that the present invention is distinct from the prior art in which gold and silver cyanides can provide, respectively, only gold and silver coatings; where the presence of ruthenium in the bath will give only blue-gray coatings; from so-called xe2x80x9ccoloredxe2x80x9d coatings which are in practice limited to black nickel coatings; from a combination of bath ingredients which gives only the so-called xe2x80x9ctiffany greenxe2x80x9d colored coating, from a different combination of ingredients which gives only a blue coating and from yet a different combination which gives only a brown coating. Moreover, the present invention achieves for the first time commercially viable electrodeposited nickel coatings of predetermined selected colors with the intrinsic advantages pertaining to nickel. While the present invention is not considered to be limited by any theory, it is believed that the variation in colors of the electrodeposited colored coating containing nickel is connected with the phenomenon of light interference; presuming this to be so, then the invention for the first time attains colors in electrodeposited nickel coatings making use of the phenomenon of light interference, according to which the color of the coating is related to its thickness. Moreover, the present invention makes possible for the first time commercially viable articles having lustrous metallic coatings, the color of which apparently depends on the phenomenon of light interference, in which the color is stable.